In wind tunnel testing of aerodynamic models and the like various devices are presently used to measure the friction caused by fluid flow over a surface. One such device commonly used is the hot wire transducer wherein a small diameter wire is placed on the tested surface and exposed to the fluid flow. In this device the wire is welded at each end to electrodes connected to an electrical circuit capable of measuring the change in resistance in the exposed wire. This change in resistance is measured as a function of temperature and skin friction. Problems associated with a skin friction sensor of this type include the generation of unwanted turbulence, difficulty in calibration and failure of the wire or weld connection in hot fluid flow.
Attempts have been made to reduce turbulence by partially setting the wire into the surface or using a foil, placed flatly on the surface as the transducer. Setting the small diameter wire into the surface is difficult and does not eliminate all turbulence. A foil sufficiently thick to carry the current required for all operating temperatures also does not eliminate turbulence.
The welding of the wire to the electrodes changes the resistance of the wire and produces a requirement for individual calibration. The wire failure problem has been overcome in one prior art device (U.S. Pat. No. 4,024,761 to Djorup) by providing support elements which add to the turbulence. Weld failure can be overcome by annealing to increase strength but this changes resistivity and produces a requirement for individual calibration. There is thus, a definite need in the art for a skin friction sensor that is turbulence free, capable of operating in a wide range of temperatures and that does not require individual calibration.
It is therefore an object of this invention to provide a skin friction sensor that does not generate turbulence in the fluid flow over a surface.
Another object of this invention is to provide an improved skin friction sensor that is resistent to high temperatures and will not fail in hot fluid flow.
A further object of this invention is to provide a skin friction sensor that does not require individual calibration.
An additional object is to provide an electrical transducer skin friction sensor capable of carrying sufficient current for all operating temperatures.
A still further object of the present invention is an improved method of making a turbulence resistant skin friction sensor.
These and other objects of the present invention together with the advantages attendant therewith will be more readily apparent when the specification is taken in conjunction with the attached drawings to which it relates.